The Testament of Sister New Devil

Toujou Basara's life changes after his father's sudden remarriage, introducing him to his two new step-sisters, Naruse Mio and Maria. Mio takes on the role of waking him up each morning, while Maria is often seen in the kitchen wearing revealing clothing.

Mio, the daughter of a former demon lord, lost her foster parents to powerful demons after surviving an attack on her life. Basara, who seeks to protect her, reveals himself as a member of the hero clan, a long-standing enemy of demons, and takes Mio in as part of his family to keep her safe.

Nonaka Yuuki, Basara's childhood friend and a member of the hero clan tasked with monitoring Naruse Mio, warns him to distance himself from her, claiming she will cause great disaster. Basara, however, refuses to back down, vowing to protect Mio as his own.

Basara is gravely injured during a battle with a white-masked man following an attack by Mio. Guilt-ridden over involving Basara, Mio confronts the masked man alone, while Basara prepares to unleash a forbidden technique he had long sealed away. The episode explores the tragic past that once devastated Basara's hero village.

After resolving the conflict with the white-masked man, the group returns to their usual chaotic lives, filled with comedic and awkward encounters. Basara finds himself in several embarrassing situations involving Mio and Yuki, while hints emerge about a hidden secret in his master-servant pact with Mio. His childhood friend Yuki also invites him on a date, adding to the growing complexity of his relationships.

Basara goes on a date with Yuki, but when they are overwhelmed by an aggressive sales pitch at a shopping center, Mio steps in to help them out.

Basara and Mio strengthen their master-servant bond as they confront Takashi and Kurumi, who seek to eliminate Mio. As Basara pushes Mio to greater heights through the use of syrup, they prepare to face their enemies with full force.

Ten days after the battle with Takashi and the others, which ended in reconciliation, Yuki is expected to return to the hero's village. However, upon returning home, Basara is surprised to find Yuki there, wearing an apron.

Basara and Yuki begin a ritual to establish their pact. Dressed in a revealing outfit, Yuki is eager to proceed but insists on altering the traditional method, resulting in her being cursed similarly to Mio.

Mio is captured by Zolgia after being betrayed by Maria, and Basara is also taken by Lars and brought to Zolgia's mansion. Lars attempts to negotiate Basara's release in exchange for Mio, claiming he is only fulfilling his duty as Mio's observer, but the talks break down and Lars is killed by Zest.

Basara, wielding the power passed down from his father, confronts the formidable Zolgia, driven by his determination to protect his loved ones. Despite his unwavering resolve and the strength it grants him, Basara finds himself pushed to his limits by Zolgia's overwhelming power. As his strength fades, he struggles to break free from Zolgia's grasp.

Basara and the others find themselves trapped within the body of a powerful demon realm creature capable of endless replication, threatening to consume the world. Cut off from their allies, they must locate and destroy the creature's vital core to stop its spread and survive.

Following the defeat of Zolgia, Basara returns to a quiet life with his two younger sisters, Mio and Maria. As he focuses on preparations for the school sports festival, he is suddenly attacked by a person under magical influence, sparking suspicions about someone he noticed at school.

Basara and his friends participate in the four-legged race at the school sports festival, enjoying the moment despite his lingering concerns about a magical threat. Meanwhile, Kurumi spots a suspicious figure, and a sudden tornado erupts on the field, forcing Basara into action to protect everyone from an unexpected new arrival.

Ornis takes Kurumi hostage and severs Basara's arm, which holds the weapon Bynhildr, causing him to lose consciousness. His repressed power awakens, leading to a violent outburst that turns the battle in his favor. Just as Basara attempts to kill Ornis, a golden chain restrains him, revealing the presence of Hasegawa Chisato, whose hidden motives come into question.

As tensions rise between the peaceful faction and the demon lord's forces, Basara and Mio visit the peaceful faction, where Basara reunites with Zest and Shella. Maria, however, behaves strangely, having betrayed the faction and now facing a severe punishment. Basara intervenes and takes responsibility for administering the penalty in place of Rukia.

Ramsus seeks to harness the power within Mio, drawing the ire of Basara, who confronts him over his disregard for Mio's well-being. Before the confrontation escalates, Shella intervenes, taking Basara to the bath in an attempt to soothe his anger and enlisting Zest to assist in the process. Shella then tries to facilitate a master-servant bond between Basara and Zest.

Basara and his companions are summoned to Leohart's royal castle by Lars, where they meet Basara's father, Toujou Jin, who delivers a message announcing a tournament to determine control of the demon realm—effectively control over Mio's power. Basara is chosen as a representative and begins training with Jin to learn how to win.

Basara and his companions confront the current demon lord, Leohart, who confidently receives them without any deceptions. With Maria's encouragement, Basara directs Mio, Yuki, Maria, Kurumi, and Zest to prepare for a ritual involving aphrodisiac-laced items, leading to a series of intimate and suggestive interactions.

The battle between Leohart and Ramsus's forces commences, with Mio securing victory in the first match. Maria then faces Lars in the next round, relying on her raw strength, but struggles against his superior experience and magic. As the demons set a trap for Kurumi, Maria must find a way to turn the tide of her fight.

The final match of the tournament pits Basara against Leohart in a climactic duel. After emerging from hiding, Basara struggles to match Leohart's superior strength, unable to rely on his usual Banishing Shift technique. As the battle intensifies, Basara must resort to an unexpected strategy to level the playing field.

The council's actions trigger violent tremors across the demon realm, awakening the powerful demon Chaos through a massive magic circle. Chaos, the commander of all heroic spirits, descends in a fury, targeting Basara, Mio, and Leohart. Meanwhile, Maria, Yuki, and their allies confront their personal struggles as they prepare for a final showdown.